The pulmonary airway comprises those parts of the respiratory system through which air flows, conceptually beginning (on inhalation from the external environment) at the nose and mouth, and terminating in the alveoli. From the mouth or nose, inhaled air passes through the pharynx into the trachea, where it separates into the left and right main bronchi at the carina, situated at the level of the second thoracic vertebra. The main bronchi then branch into large bronchioles, one for each lobe of the lung. Within the lobes, the bronchioles further subdivide some twenty times, ending in clusters of alveoli.
The epithelial surfaces of the airway contain cilia. Inhaled particles adhere to mucus secreted by goblet cells, which is continuously driven outwards by the cilia. The epithelium of the airway also secretes a watery fluid upon which the mucus can ride freely. The production of this fluid is impaired by the disease cystic fibrosis. Macrophages in the airways help promote prophylaxis and prevent infection and contamination, by engulfing bacteria and other inhaled particles.
Disease conditions associated with the airway include cystic fibrosis, allergies, asthma, Chronic Obstructive Pulmonary Disease (COPD) and bronchitis. Cystic fibrosis (also known as CF or mucoviscidosis) is a common recessive genetic disease which affects the entire body, causing progressive disability and often early death. The name cystic fibrosis refers to the characteristic scarring (fibrosis) and cyst formation within the pancreas, first recognized in the 1930s. Difficulty breathing is the most serious symptom and results from frequent lung infections that are treated with, though not cured by, antibiotics and other medications. A multitude of other symptoms, including sinus infections, poor growth, diarrhea, and infertility result from the effects of CF on other parts of the body.
Currently, there are no cures for cystic fibrosis, although there are several treatment methods. The management of cystic fibrosis has improved significantly over the years. While infants born with cystic fibrosis 70 years ago would have been unlikely to live beyond their first year, infants today are likely to live well into adulthood. The cornerstones of management are proactive treatment of airway infection and inflammation, and encouragement of good nutrition and an active lifestyle. Management of cystic fibrosis is aimed at maximizing organ function, and therefore quality of life. At best, current treatments delay the decline in organ function. Targets for therapy are the lungs, gastrointestinal tract (including pancreatic enzyme supplements), the reproductive organs (including assisted reproductive technology (ART)) and psychological support.
The most consistent aspect of therapy in cystic fibrosis is limiting and treating the lung damage caused by thick mucus and infection, with the goal of maintaining quality of life. Intravenous, inhaled, and oral antibiotics are used to treat chronic and acute infections. Mechanical devices and inhalation medications are used to alter and clear the thickened mucus. These therapies, while effective, can be extremely time-consuming for the patient. One of the most important battles that CF patients face is finding the time to comply with prescribed treatments while balancing a normal life.
Small-interfering RNA (siRNA)-mediated silencing of genes offers a novel approach for disease treatment. Direct delivery of siRNA to respiratory epithelia is potentially advantageous for many respiratory infections and for chronic diseases like cystic fibrosis where airway epithelial cells are prominent sites of production and release of pro-inflammatory cytokines such as IL-8 and others (Davidson B L, McCray P B, Jr. Current prospects for RNA interference-based therapies. Nat Rev Genet 2011; 12(5):329-340). Topical delivery avoids hepatic clearance and non-specific accumulation associated with the systemic route and allows for local accumulation within the target organ. But due to its high molecular weight and polyanionic nature, siRNAs do not cross the epithelial cell membrane freely. In addition, the intra pulmonary physical barriers such as mucus to overcome before encountering the problems with cell entry (Oakland M, Sinn P L, McCray P B Jr. Advances in cell and gene-based therapies for cystic fibrosis lung disease. Mol Ther. 2012 Feb. 28. doi: 10.1038/mt.2012.32. [Epub ahead of print] PMID: 22371844). Thus, efficient delivery of siRNA to the airways has been challenging due to significant intracellular and extracellular barriers.
Non-viral siRNA delivery is an attractive and potentially safer alternative to virus-based delivery systems. A number of studies report successful delivery of naked siRNA to airways, especially for counteracting viral infections (Zhang W et al., Inhibition of respiratory syncytial virus infection with intranasal siRNA nanoparticles targeting the viral NS1 gene. Nat Med 2005; 11(1):56-62; Bitko V et al., Inhibition of respiratory viruses by nasally administered siRNA. Nat Med 2005; 11(1):50-55). However, recent reports also show that siRNAs delivered intranasally or intratracheally, without delivery enhancement, may not target to lung cells and thus do not cause RNA interference (Moschos S A et al., Uptake, efficacy, and systemic distribution of naked, inhaled short interfering RNA (siRNA) and locked nucleic acid (LNA) antisense. Mol Ther 2011; 19(12):2163-2168). Furthermore, off target immunostimulatory effects of early siRNA constructs likely clouded some studies (Judge A D et al., Sequence-dependent stimulation of the mammalian innate immune response by synthetic siRNA. Nat Biotechnol 2005; 23(4):457-462; DeVincenzo J et al., A randomized, double-blind, placebo-controlled study of an RNAi-based therapy directed against respiratory syncytial virus. Proc Natl Acad Sci USA 2010; 107(19):8800-8805; Kleinman M E et al., Sequence- and target-independent angiogenesis suppression by siRNA via TLR3. Nature 2008; 452 (7187):591-597). Added to these disappointing results, the delivery and efficacy of siRNA in combination with various non-viral reagents in respiratory epithelia has not been extensively investigated.
Accordingly, a more effective, simple-to-administer, and efficient treatments for CF and other airway diseases are needed.